Cables for transmitting high-frequency currents



April 71964 R. c. MILDNER 3,130,256

CABLES FOR TRANSMITTING HIGH-FREQUENCY CURRENTS Filed June 50, 1961 2Sheets-Sheet 1 PbLYETHYLENE COPOLYMER 5 TO 20 MIL. THICK POLYETHYLENEINVENTOR RAYMOND CHARLES MILDNER ATTORNE April 21, 1964 R. c. MILDNER3,130,256

CABLESFOR TRANSMITTING HIGH-FREQUENCY GURRENTS Filed June 30, 1961 2heetsh 2 LAYER 5 TO 20 MIL.THICK INVENTOR RAYMOND CHARLES MILDNER UnitedStates Patent 3,130,256 CABLES FOR TRANSMITTING HIGH-FREQUENCY CURRENTSRaymond Charles Mildner, 3703 Hillgrove Court,

I Midland, Mich.

Filed June 30, 1961, Ser. No. 121,128 Claims priority, application GreatBritain July 4, 1960 .10 Claims. (Cl. 174-28) This invention relates tocables for transmitting highfrequency currents and is primarilyconcerned with but not limited to the return or outer conductor ofaflexible co-axial'cable designed to transmit currents of a frequency ofabout 1 megacycle per second or higher.

In flexible cables used for transmitting such high-frequency currents itis customary to use as the insulation, separating the go and returnconductors, a substantially solid flexible insulating material. In othercases a semiair-spaced insulation may be used, such as that provided byan extruded synthetic plastic material of cart-wheel cross-section, ahelical thread of plastic surrounded by an extruded plastic tube or byextruded cellular plastic. The common feature of all of theseconstructions is that the insulation is suflicientlyrobust to supportthe return conductor or screen.

The return conductor is usually in the form of a coaxial cylindricalbraid of copper wires, which may be plain, tinned or silver-plated. Incases where the requirements of flexibility are less stringent, thisreturn conductor may be in the form of a thin-walled cylinder, providedwith transverse corrugations to afford a measure of flexibility and toavoid kinking when the cable is bent. Since hermetic sealing is notessential with a substantially solid insulation, the cylindricalconductor is usually formed from a tape which is folded longitudinallyto form a tube, the edges of the tape being eitherbutted or overlapped.Corrugation of the conductor may be effected either before or after thetape has been formed into the tube.

This form of return conductor has the advantage over a braided conductorthat it has a'very much lower electrical resistance at high frequenciesthan an equivalent braided conductor, so that the signal which has to betransmitted by a cable with this form of return conductor is attenuatedless than if the return conductor is a braided one. A return conductorformed of a corrugated tape will rarely have a resistance at highfrequencies greater than 30% above that of a straight cylindricalconductor of similar material and of the same size, whereas a braidedconductor will usually have an impedance of from .2 to 4 times that of acomparable straight cylindrical conductor.

The corrugated tape is also a much more eifective screen againstexternal electro-magnetic fields than is a braid, particularly at veryhigh frequencies, so that it can often replace a double-braided cable.The corrugated tape construction is also attractive economically.

The weakness of the corrugated tape as a return conductor or screen liesin its mechanical properties. It is not possible to bend the cable rounda very small radius without disastrous kinking of the tape. To avoiddeformation due to crushing and to minimise kinking, it is necessary touse a'fairly thick tape, usually of about 10 mils (l mil=0.001 inch) inthickness, but sometimes a little thinner. This in turn makes the cableless easy to bend and increases the cost.

According to the present invention there is provided a conductor for acable, for transmitting high-frequency currents, comprising a corrugatedtube formed from a laminate whichcomprises a layer of a syntheticplastic material having a thickness of from 5 to 20 mils, and anelectrically conducting metallic layer having a thickness of from M2 to3 mils, the tube being formed by longitudinally folding a tape of thelaminate. The conductor of formed into the tube.

the present invention is especially useful as either the outer or returnconductor of a coaxial cable or the go or inner conductor of such acable. Furthermore, the screen of a balanced cable for transmitting highfrequency currentsin which go and return conductors lie side-bysi de maybe a conductor of the present invention.

If desired, the conducting tape may be sandwiched between two tapes ofsynthetic plastic material, or, alternatively, the laminate may befurther combined with a plain uncorrugated tube of synthetic plasticmaterial so as to provide increased longitudinal strength androbustness.

In its application to a cable, the corrugated tape may be applied withthe conducting element either on the outside or on the inside of thecable. In the former case the plastic tape forms part of the insulationand its electrical properties must be reasonably good. When theconducting element is placed on the inside, the plastic tape providesadditional mechanical protection to the conducting element or may alsoserve as a separator or insulating barrier to a second conductingscreen.

The preferred material for the conducting tape is copper, as it providesthe best compromise in regard to electrical conductivity, mechanicalproperties, and cost. Other suitable materials are aluminium and silver.Plated or laminated conductors may be used to impart special propertiesto the conductor, such as corrosion resistance or resistance to hightemperatures.

The synthetic plastic material of which the tape is made is preferablypolyethylene, polypropylene or polystyrene, or laminates of thesematerials with re-inforcing elements such as glass-fibre weave or tape.Other materials may be used, the main requirements being high breakingstrength, an elongation to break greater than 30% and a modulus ofelasticity which is reasonably high but very low in comparison with thatof the conducting element.

As indicated above, the conductor is formed from 21 tape comprising alaminate, the tape being longitudinally folded to form a tube, and theedges of the tape being either butted or overlapped. Corrugation of theconductor may be eliected either before or after the tape has been Ithas been found, however, that when the laminate is passed at roomtemperature through tube-forming rolls or corrugating rolls there is anexcessive amount of elastic recovery of the deformed material. In orderto reduce the elastic recovery, it is desirable to heat the laminate toa temperature just below the melting point of the synthetic plasticmaterial since plastic deformation of the material will be great at thistemperature. This heating operation is rendered possible because of thepresence of the metal layer which supports the synthetic plasticmaterial.

In order to enable the invention to be more readily understood,reference will now be made to the accompanying drawings which illustratediagrammatically and by way of example some embodiments thereof, and inwhich:

FIGURE 1 is a cut-away view of part of a co-axial cable,

FIGURE 2 is a cross-section of the cable along the line IIII in FIGURE1.

FIGURES 3 and 4 are views similar to FIGURE 1 showing modifiedembodiments of the cable of FIG- there is shown a co-axial cablecomprising a solid copper conductor 1 having a diameter of 0.048", andconstituting the inner or go conductor of the cable. The conductor 1 iscovered with solid polyethylene insulation 2' having a diameter of0.240". The outer or return conductor 3 of the cable is formed from alaminate consisting of a layer of copper foil 4 having a thickness of 1mil, and a layer of a polyethylene copolymer 5 having a thickness of 10mil. The return conductor is corrugated with a pitch of 0.150" to asemi-amplitude of 0.020, and, as shown in FIGURE 2, the conductor isformed from a tape which is folded longitudinally to form a tube, theedges being overlapped.

The cable has an attenuation coefficient which is at least 15% lowerthan that of a copper-braided cable of the same overall size.

If desired, the cable may be provided with an overall sheath ofpolyvinyl chloride to provide mechanical protection and protectionagainst corrosion.

In cases where, the air space 6 underneath the corrugated conductor 3 isundesirable, this space may be filled with a material 7 which is plasticbut form-stable at the operating temperatures of the cable as shown inFIGURE 3. Alternatively, the surface of insulation 2 may be grooved asshown at 8 in FIGURE 4 so that the corrugated laminate constituting theouter conductor 3 conforms to the surface of the core. In this lattercase, the outer conductor may be first formed into the cylindrical tube,and then pressed into the grooves of the insulating core.

If desired, the conducting tape 4 may be sandwiched between two tapes 5and 9 of synthetic plastic material as shown in FIGURE 5, oralternatively the laminate may be combined with a plain uncorrugatedtube 10 of synthetic plastic material so as to provide increasedlongitudinal strength and robustness.

While the invention has been described as being applicable to the outeror return conductor of a flexible coaxial cable, it is also applicableto the screen 11 of balanced cables, as shown in FIGURE 7, in which theg and return conductors indicated as 12 and 13 in FIG- URE 7, lieside-by-side. It is also within the purview of this invention to providethe overall protective sheath 14 which may be of polyvinyl chloride.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

1. A conductor for a cable for transmitting high frequency currents,comprising a corrugated tube formed from a laminate which comprises alayer of synthetic plastic material having a thickness of from to 20mil, and an electrically conducting metallic layer laminated to theplastic layer and having a thickness from /2 to 3 mil and an additionallayer of synthetic plastic material, the metallic layer being sandwichedbetween the layers of synthetic plastic material, and the tube having ajoint formed by the edges of the laminate extending longitudinally ofthe tube and parallel to the axis thereof.

2. The conductor of claim 1, wherein the synthetic plastic material is athermoplastic material selected from the group consisting ofpolyethylene, polypropylene and polystyrene.

3. The conductor of claim 1, wherein the second mentioned syntheticplastic material is laminated to an exterior sheath of polyvinylchloride.

4. A conductor for a cable for transmitting high frequency currents,comprising a corrugated tube formed from laminate which comprises alayer of synthetic plastic material having a thickness of 5 to 20 millaminated to an electrically conducting metallic layer having athickness of from /2 to 3 mil and a plain uncorrugated tube positionedwithin the corrugated tube with the outer surface of said plain tubeengaging the crests of the corrugations of the corrugated tube, theconductor having a joint extending longitudinally of the conductorparallel to the axis thereof.

5. A coaxial cable for transmitting high frequency currents comprisingan inner conductor, an outer conductor, and insulation materialseparating the two conductors, at least one of said conductorscomprising a corrugated tube formed from a laminate which comprises alayer of synthetic plastic material selected from the group consistingof polyethylene, polypropylene and polystyrene and having a thickness offrom 5 to 20 mil, said layer being laminated to an electricallyconducting metallic layer selected from the group consisting of theelectrically conducting copper, aluminium and silver, and having athickness of from /2 to 3 mil, the conductor tube having a jointextending longitudinally of the tube parallel to the axis thereof.

6. The cable of claim 5, wherein said joint is a lap joint.

7. The cable of claim 5, wherein the corrugated tube conductor isarranged so that the electrically conducting metallic layer is insidethe synthetic plastic layer.

8. The cable of claim 5, wherein said corrugated tube constitutes theouter conductor, and wherein the insulation material fills the spacebetween the outer conductor and the inner conductor.

9. The cable of claim 5, wherein the layer of insulating material isgrooved and wherein the corrugated tube constituting the outer conductorconforms to the surface of the insulating material.

10. A balanced cable for transmitting high frequency currents,comprising a go conductor and a return conductor lying side-by-side, anda screen for the conductors comprising a corrugated tube formed from alaminate which comprises a layer of synthetic plastic material having athickness of from 5 to 20 mil and an electrically conducting metalliclayer laminated to the plastic layer and having a thickness of from /2to 3 mil, the tube having a joint extending longitudinally thereofparallel to the axis thereof.

References Cited in the file of this patent UNITED STATES PATENTS2,172,978 Kirch Sept. 12, 1939 2,348,752 Quayle May 16, 1944 2,436,421Cork Feb. 24, 1948 2,447,168 Dean et al Aug. 17, 1948 2,614,172Greenfield et al Oct. 14, 1952 2,808,450 Peters Oct. 1, 1957 2,939,905Canfield June 7, 1960 2,960,561 Plummer Nov. 15, 1960 FOREIGN PATENTS752,006 France July 3, 1933 932,619 Germany Sept. 5, 1955 955,331Germany Jan. 3, 1957

5. A COAXIAL CABLE FOR TRANSMITTING HIGH FREQUENCY CURRENTS COMPRISINGAN INNER CONDUCTOR, AN OUTER CONDUCTOR, AND INSULATION MATERIALSEPARATING THE TWO CONDUCTORS, AT LEAST ONE OF SAID CONDUCTORSCOMPRISING A CORRUGATED TUBE FORMED FROM A LAMINATE WHICH COMPRISES ALAYER OF SYNTHETIC PLASTIC MATERIAL SELECTED FROM THE GROUP CONSISTINGOF POLYETHYLENE, POLYPROPYLENE AND POLYSTYRENE AND HAVING A THICKNESS OFFROM 5 TO 20 MIL, SAID LAYER BEING LAMINATED TO AN ELECTRICALLYCONDUCTING METALLIC LAYER SELECTED FROM THE GROUP CONSISTING OF THEELECTRICALLY CONDUCTING COPPR, ALUMINUM AND SILVER, AND HAVING ATHICKNESS OF FROM 1/2 TO 3 MIL, THE CONDUCTOR TUBE HAVING A JOINTEXTENDING LONGITUDINALLY OF THE TUBE PARALLEL TO THE AXIS THEREOF.